In the area of structures of semiconductor-on-insulator (SOI) type, it is known to apply thermal treatment to cause diffusion of at least part of the oxygen from the buried oxide layer through the thin semiconductor layer, so as to reduce or eliminate the thickness of this oxide layer.
This dissolution step can be applied to the entire structure, or it can be applied locally, i.e., to dissolve the oxide layer—in whole or in part—in determined regions of the SOI structure, corresponding to a desired pattern, while preserving the initial oxide layer in the other regions. This is termed “local dissolution” of the oxide layer.
The application of this dissolution thermal treatment or of any other type of thermal treatment, such as surface smoothing or an epitaxy step, may cause a phenomenon of de-wetting as mentioned in document WO 2007/048928. It was observed by the inventors of the present invention that this phenomenon was particularly triggered on the periphery of the structure and, more generally, at every point in the structure in which the buried oxide is exposed, i.e., in contact with the outside, or at every point in which the thin layer is so thin (a few tens of nanometers or less) that it becomes morphologically unstable when the structure is brought to high temperature, thereby exposing the buried oxide.
This de-wetting phenomenon is greater the thinner the thickness of the thin layer (for example, less than 100 nm).
It is accompanied by a phenomenon of etching of the buried oxide if this oxide is exposed to the treatment atmosphere and in contact with the thin layer in Si, for example, as per the reaction:SiO2+Si→(gaseous)2SiO
At the “three-point” contact of Si/SiO2/treatment atmosphere, the buried oxide (SiO2) reacts with the silicon of the thin layer to form volatile SiO complexes, which are carried into the treatment atmosphere.
It is recalled that in substrates of semiconductor-on-insulator (SOI) type obtained by bonding, there is no bonding on the edge of the assembled substrates on account of the presence of peripheral chamfers on the contacted surfaces. Therefore, after transfer of the thin layer, in silicon, for example, the final substrate has a peripheral ring in which transfer did not take place and at which the buried oxide is exposed.
Phenomena of de-wetting and etching, which may occur during thin layer thermal treatments, lead to an extensive defective region over a distance of up to 1 cm on the periphery of the substrate when this substrate is exposed to thermal treatment, e.g., dissolution thermal treatment.
This defectiveness may also occur at any other point of the starting substrate, at which the buried oxide is directly exposed or lies under a thin layer of reduced thickness. The defect is possibly a through defect (also called “HF defect”) in the thin layer and, therefore, exposes the buried oxide to the treatment atmosphere. It may concern regions of the substrate in which the thin layer is of much narrower thickness than the mean thickness of this layer, making exposure of the buried oxide to the treatment atmosphere possible if the thickness of the thin layer should be reduced during treatment.
These phenomena make the substrate unfit for uses in the usual areas of application of these substrates, such as the fabrication of electronic components (e.g., “memory” and “logic” components), in the regions in which such defects occur. Thus, improvements in these types of components are needed.